Pulping of wet strength paper with hypochlorous acid



United States Patent Ofice 3,407,l l3 Patented Oct. 22, 1968 3,407,113 PULPING OF WET STRENGTH PAPER WITH HYPOCHLOROUS ACID Charles Sidney Maxwell, South Charleston, W. Va., as-

signor to American Cyanamid Company, Stamford, 'Conn., a corporation of Maine No Drawing. Continuation-impart of application Ser. No. 289,404, June 20, 1963, which is a continuation-in-part of application Ser. No. 8,912, Feb. 16, 1960. This application Mar. 3, 1966, Ser. No. 531,378

2 Claims. (Cl. 1626) This is a continuation-in-part of copending applications Ser. No. 8,912 and 289,404, filed by me on Feb. 16, 1960, and June 20, 1963 and now abandoned.

The present invention relates to the pulping of wetstrength paper. More particularly, the invention relates to the pulping of paper broke having wet-strength properties by reason of a content of a polyalkylenepolyamine resin.

US. Patent No. 2,872,313, granted on Feb. 3, 1959, to R. R. House et al., discloses that wet-strength paper scrap or broke is more easily pulped after it has been contacted with a dilute aqueous solution of an oxidizing salt. The solution loosens the fiber-resin bonds sufficiently so that the paper can be pulped in conventional mechanical pulping equipment, without need for exposure to heat that otherwise would be necessary. It is a disadvantage of the process, however, that the treating agents are salts, so that directly or indirectly the process requires the supply of two components viz., an acid and a base.

The discovery has now been made that the pulping of paper composed of cellulose fibers bonded together by an adsorbed content of a substantially linear polyalkylenepolyamine wet-strength resin is greatly facilitated by contacting the paper with an aqueous solution of hypochlorous acid (HOCl). The hypochlorous acid is substantially as effective in weakening the fiber-resin bonds as the oxidizing salts disclosed in said patent.

According to the invention, the paper is slurried with an aqueous solution of hypochlorous acid in the substantial absence of metal cations until the fiber-resin bonds have loosened sufficiently to render the paper pulpable by mechanical means.

The invention does not depend primarily upon the strength of the hypochlorous acid solutions employed. Since substantially no metal cations are present, the pH of the solutions is less than 7.

Because of its ease of preparation and cheapness, I prefer the mixtures of hypochlorous and hydrochloric acids formed by bubbling or otherwise introducing chlorine gas into water at about atmospheric pressure. Such mixture of acids is generally termed chlorine water.

The hypochlorous acid content of chlorine water prepared by subst-antially saturating water with chlorine as described above is only about /s% by weight, and it is surprising that rapid and extensive loosening of the fiberresin bonds is produced by acid at this low concentration. However, I have found that solutions of one-tenth this strength and less are very useful, very satisfactory bond loosening being effected in a somewhat but not unduly longer time, and where facilities are available for overnight treatment of the paper, hypochlorous acid solutions containing as little as 50 parts per million by weight of hypochlorous acid have been found useful.

If desired, the hypochlorous acid solution may be prepared by other means including acidification of an aqueous solution of Ca(OCl) with at least one equivalent of sulfuric acid, so that substantially all of the calcium is precipitated as calcium sulfate.

It is preferred to treat the paper in the temperature range of 20 C.40 C. In this range the hypochlorous acid acts rapidly, and loss by evaporation is held to a low figure.

Laboratory experiments have shown the process of the present invention to be of substantial value in the pulping of wet strength paper wherein the strengthening agent is a normally water-soluble polyalkylenepolyamine, or a substantially linear resin having a combined polyalkyenepolyamine content of more than 10% by weight, herein termed polyalkylenepolyamine resin.

Polyalkylenepolyamine resins include polyethylenimine itself (substantially composed of consecutive aminoalkylene NH-CH -CH linkages), and also the resins containing interrupted or nonconsecutive aminoalkylene linkages. Such resins may be prepared by reacting a lower polyalkylenepolyarnine first with coupling agents such as methylene-bisacrylamide, adipic acid, adipoyl chloride, N,N-di(2-hydroxyethyl)adipamide and di-Z-hydroxyethyl adipate, to form high molecular weight water-soluble linear polymers and then reacting with 1,3-dich1oropropane, epichlorohydrin, etc., to build up the molecular weight. The resins are cross-linked. Suitable lower polyalkylenepolyamines for the preparation of the above resins include diethylenetriamine, tetraethylenepentamine, 3,3'- iminobispropylamine, the polypropylenepolyamines, and the polyalkylenepolyamines made by reacting alkylidene or arylenedihalides for example 1,3-dichloropropane or 1,3-dichlorobenzene with lower alkylenediamines for example ethylenediamine and trimethylenediamine.

Paper wherein the fib'ers are bonded together by resins of other types have been benefited little by the treatment.

During the process, hypochlorous acid is constantly lost by reaction with the wet strength resin present and with any other oxidizable material in the paper. Accordingly, for most rapid pulping with papers such as unbleached northern kraft pulp which contains a high proportion of oxidizable material, it will often be found desirable to maintain the hypochlorous acid level more or less constant in any convenient way during the treatment, for example, by bubbling chlorine gas slowly into the paper slurry.

The following constitutes specific embodiments of the invention and are not be construed as limitations thereon.

EXAMPLE 1 The following illustrates the effect of a very small amount of hypochlorous acid as pulping agent on wet strength paper containing several polyalkylenepolyamine resins, shown in comparison with the results obtained with wet strength paper containing resins of other types.

The tests were made by immersing strips of standard wet strength paper /z" x 4" (basis weight 50' lb. per x "/500 ream) containing the resins shown below for 5 minutes in chlorine water at 73 F. In each instance 250 cc. of chlorine water containing approximately 0.1% HOCl by weight was employed. The solutions were not stirred during the test, nor was the HOCl consumed replaced. Wet strength values were obtained before and after the test; a value of less than 3 1b. shows that the resin-fiber bonds are sufficiently weak to permit'the paper to be pulped by mechanical means. Results are as follows:

ethylene-diamine.

I Made by process of Maxwell et al. U.S. Patent No. 2,559,220. 1 Made by process of Daniels ct a1. U.S. Patent No. 2,657,132.

5 Made by process of U.S. Patent No. 2,885,315.

Made by process of Canadian Patent No. 473,753 (Ex. 3).

5 Made by process of U.S. Patent N 0. 2,882,185.

6 Molecular weight about 50,000.

7 Made by process of Lundberg et a1. copending appu. S.N. 655,488 filed April 29, 1057 now U.S. Patent No. 3,019,156.

The resins in the papers of Runs 1-4 are not polyalkylenepolyamine resins and are negligibly weakened by the chlorine water solution. The resins in the papers of Runs 5-7 are polyalkylenepolyamine resins and the papers are weakened to the point where they can be pulped by ordinary beating equipment.

EXAMPLE 2 The procedure of Examples 5, 6 and 7 of Example 1 is repeated, except that all strips are immersed in chlorine water (prepared by mixing 100 cc. of saturated chlorine water with 900 cc. of plain water) containing 0.01% by weight of HOCl and the duration of treatment is 4 hours. The estimated strength of the strips is less than 3 lb./ inch.

I claim:

1. Process for thepulping of paper composed of, cellulose fibers bonded together by an adsorbed content of at least one polyalkylen'epolyamine wet strength resin which comprises contacting said paper with an aqueous solution of chlorine water until the fiber-resin bonds in said paper have loosened, and then subjecting the paper to mechanical pulping.

2. Process for the pulping of paper composed of cellulose fibers bonded together by an adsorbed content of at least one polyalkylenepolyamine wet strength resin, which comprises contacting said paper with an aqueous solution of hypochlorous acid in the substantial absence of dissolved metal cations at a pH below 7 until the fiberpolyalkylenepolyamine bonds in said paper have loosened, and then subjecting said paper to mechanical pulping, the hypochlorous acid content of the solution being maintained in excess of parts per million by passing gaseous chlorine into said solution.

References Cited UNITED STATES PATENTS 2/1959 House 1626 OTHER REFERENCES DONALL H. SYLVESTER, Primary Examiner.

H. R. CAINE, Assistant Examiner. 

1. PROCESS FOR THE PULPING OF PAPER COPOSED OF CELULOSE FIBERS BONDED TOGETHER BY AN ADSORBED CONTENT OF AT LEAST ONE POLYALKYLENEPOLYAMINE WET STRENGTH RESIN SOLUTION OF CHLORINE WATER UNTIL THE FIBER-RESIN BONDS IN SAID PAPER HAVE LOOSENED, AND THEN SUBJECTING THE PAPER TO MECHANICAL PULPING. 